on to gain length by
repeating each half--in the early days of the form, _literally_ (with
a double bar and sign of repeat); later, as composers gained freedom,
with considerable amplification. Each half presented the _same_
material (it was a _one_-theme form) but the two halves were
contrasted in _tonality_, _i.e._, the first part, beginning in the
home-key, would modulate to some related key--generally the dominant;
the second part, starting out in this key, gradually modulated back to
a final cadence in the original key, and often--especially in Haydn
and Mozart--repeated the entire main sentence of the first part. The
general effect of such a form has been wittily described[65] as
resembling the actions of "the King of France who, with twenty
thousand men, marched up the hill and then marched down again"--but he
surely had no exciting adventures in between! It is evident that this
form, while favorable to coherence and unity, is lacking in scope and
in opportunity for variety and contrast. It did, however, emphasize
the principle of recapitulation; in fact it became the convention (as
we shall see in the dances of the Suite) for the closing measures of
the second part to be an exact duplicate in the home-key of that which
had been presented at the end of part one. We shall observe, as we
continue our studies, that the trend of musical composition gradually
swung over to the Three-part form, the essential feature of which is
restatement after _intervening contrast_.

[Footnote 65: See _The Appreciation of Music_ by Surette and Mason, p.
36.]

For illustrations of the Two-part Form see the Supplement Nos. 20, 21,
22, 23, 24.

Only in such comparatively simple examples as those just cited is
found this perfect balance in the length of the two parts. We often
observe extended sentences in the first part; and it became the custom
for the second part to be considerably lengthened, to include
modulations into more remote keys and even to display certain
developments of the main material. For a striking example of a
movement which, although definitely in Two-part form, (_i.e._, it is
in two clear divisions and has but _one_ theme) is yet of considerable
scope and variety, see the Allegretto of Beethoven's Fourth Sonata. It
was, in fact, this instinct for contrasting variety in the second
part[66] which (as can be shown from historical examples)[67]
gradually led to the developing and establishment of the Three-part
fo